1. Field of Inventions
The present inventions relate generally to neurostimulation systems.
2. Description of the Related Art
Neurostimulation systems, such as spinal cord stimulation (SCS) systems, deep brain stimulation systems and subcutaneous stimulation systems, include electrodes that are positioned adjacent to neural elements that are the stimulation target. The electrodes are commonly mounted on a carrier and, in many instances, a plurality of electrodes are mounted on a single carrier. These carrier/electrode devices are sometimes referred to as “leads.” Because the proper placement of the electrodes is critical to the success of neurostimulation therapy, the surgeon will carefully position one or more leads such that the electrodes are adjacent to the target neural elements. There will typically be 1 to 5 mm between adjacent leads.
Stimulation energy is delivered to the electrodes during and after the placement process in order to verify that the electrodes are stimulating the target neural elements. Stimulation energy is also delivered to the electrodes at this time to formulate the most effective stimulus pattern (or regimen). The pattern will dictate which of the electrodes are sourcing or returning current pulses at any given time, as well as the magnitude and duration of the current pulses. The stimulus pattern will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit (e.g. pain relief), yet minimizes the volume of non-target tissue that is stimulated. Thus, neurostimulation leads are typically implanted with the understanding that the stimulus pattern will require fewer than all of the electrodes on the leads to achieve the desired “paresthesia,” i.e. a tingling sensation that is effected by the electrical stimuli applied through the electrodes.
A wide variety of leads have been introduced. One common type of neurostimulation lead is the “in-line” lead, which includes a plurality of spaced electrodes on a small diameter carrier. In-line leads are relatively easy to place because they can be inserted into the spinal canal through a percutaneous needle in a small locally-anesthetized incision while the patient is awake and able to provide feedback. In-line leads are also advantageous because they can be removed relatively easily. One of the disadvantages of in-line leads is that they are prone to migrating in the epidural space, either over time or as a result of a sudden flexion movement.
Lead migration can result in the targeted neural elements no longer being appropriately stimulated and the patient no longer realizing the full intended therapeutic benefit. Lead migration is, however, not the only reason that the therapeutic effects of a previously effective neurostimulation regimen will diminish or simply disappear, which can make diagnosis difficult. Moreover, even after a physician has determined that lead migration has occurred and that the system must be reprogrammed to accommodate the new positions of the electrodes, conventional neurostimulation systems do not provide the physician with information about the movement of an individual lead, such as how far the lead has moved relative to the underlying tissue. This makes reprogramming especially difficult because it relies on trial and error and patient feedback to identify which of the lead electrodes are now aligned with the target neural elements and which are not.
The present inventors have also determined that conventional methods of detecting the relative positions of two or more neurostimulation leads at the time of implantation, as well as at subsequent times, are susceptible to improvement.